As the driving force control system for a vehicle such as described above, an example technique is disclosed in Japanese Laid-Open Patent Application Publication No. 2009-132195.
This driving force control system may be propelled by a drive or propulsion source other than the engine such as an electric motor. In particular, the technique is particularly applicable to a hybrid vehicle in which a first clutch capable of varying a transfer torque capacity is interposed between the engine and the drive wheels and a second clutch capable of varying a transfer torque capacity is interposed between the electric motor and the drive wheels. This is detailed below.
In accordance with a uphill or the temperature of the second clutch, the engine is allowed to rotate autonomously with the first clutch released and, by holding the motor rotation speed of the electric motor below the engine rotation speed, the driving force is realized or transferred with the second clutch being under a slip control, thereby suppressing the heat generation of the second clutch.
However, in the drive control system for a vehicle described above, the following problem arises. Specifically, in an extremely low speed region where an output rotation sensor is not able to detect the output shaft rotation speed necessary for suppressing the heat generation by the second clutch, in view of the importance to suppress the heat generation, the input rotation speed has been decreased to a region where the slip rotation of the second clutch cannot be detected while feed-forward controlling the second clutch.
Incidentally, with respect to the second clutch, since a difference may occur between a hydraulic pressure command value controlling the torque capacity thereof and an actual pressure, or the properties of the friction member of the second clutch per se change over time and undergoes aging, the control characteristic does not remain the same.
Therefore, the actual torque capacity representing the control result of the second clutch does not match the target torque capacity as a control command. The difference or deviation in both may create a difference of torque capacity.
With the conventional technique, in an extremely low speed region where an output rotation sensor is not able to detect the output shaft rotation speed necessary for suppressing the heat generation by the second clutch, in view of the importance to suppress the heat generation, the input rotation speed has been decreased to a region where the slip rotation of the second clutch cannot be detected while feed-forward controlling the second clutch.
During this feed-forward control, when the second clutch produces the deviation in torque capacity, then the driver is not in a position to achieve the desired, target driving force so that the problem of deterioration in drivability of the hybrid vehicle will arise.
On the other hand, however, despite the situation in which the output shaft rotation speed necessary for suppressing heat generated by the second clutch in the extremely low speed is not detectable by the output rotation sensor, if a driving force control is continued based on the false detection of the output rotation sensor, the calculated value of the slip rotation of the second clutch is also false so that the heat generation of the second clutch may not be suppressed as desired. On the contrary, such control would lead to promoting the heat generation. At any rate, it has been conventionally difficult to compromise the heat generation suppression of the second clutch for the vehicle drivability.